The invention relates to a method of gas detection using real time differential pulse detection ("RDPD"), to an electronic system for carrying out the method, and to an electrochemical detection cell which can be used in the system. Real time differential pulse detection (RDPD) is a modification of differential pulse voltammetry ("DPV").
Known analytical systems using conventional voltammetry maintain an accurate potential between the sensing and reference electrodes of an electrochemical sensing cell which may include, in addition to the sensing and reference electrodes, a counter electrode.
Examples of prior art electrochemical detection cells, which include a sensing electrode, a reference electrode and a counter electrode are illustrated in respective U.S. Pat. Nos. 3,776,832 and 3,925,183 to Oswin et al. entitled respectively "Electrochemical Detection Cell" and "Gas Detecting and Quantitative Measuring Device" and issued respectively Dec. 4, 1973 and Dec. 9, 1975. The electrodes are of a type which must be operated in a conventional voltammetry mode and, consequently, one cannot realize the advantages of RDPD techniques.
Another example of a known electrochemical detection cell, which includes an anode, a cathode and a reference electrode is disclosed in U.S. Pat. No. 4,201,634 to Stetter entitled "Method of Detection of Hydrazine" and issued May 6, 1980. In this instance, the sensing or working electrode includes a noble metal catalyst capable of electrooxidation of hydrazine bonded to a hydrophobic material. The electrodes are so configured that detection cells constructed as proposed have the same shortcomings as those disclosed in the Letters Patents to Oswin et al., supra; selectivity and sensitivity is limited.
Additional examples of electrochemical detection cells are disclosed and illustrated in the U.S. Pat. Nos. identified as follows:
______________________________________ U.S. Letters Pat. Nelms et al. August 9, 1977 No. 4,040,805 U.S. Letters Pat. David et al. September 13, 1977 No. 4,048,041 U.S. Letters Pat. Dempsey et al. October 14, 1980 No. 4,227,984 U.S. Letters Pat. Kring et al. November 25, 1980 No. 4,235,097 U.S. Letters Pat. Diller et al. June 2, 1981. No. 4,271,121 ______________________________________
A detailed discussion of the theory, techniques and waveforms used in conventional differential pulse voltammetry as used in laboratories can be found in Sawyer et al., Experimental Electrochemistry for Chemists, pages 280, 389-394, John Wiley & Sons, New york, N.Y. (1974) and in Skoog et al, Fundamentals of Analytical Chemistry, 3rd Edition, pages 492-494, Sanders College, Philadelphia, Pa. (1976). In this known technique, a linearly increasing D.C. voltage ramp is imposed on the sensing electrode, with respect to a reference electrode, and a small D.C. pulse is superimposed on the ramp. The difference in current at the end of the pulse and before the pulse is utilized to determine the D.C. level at which a particular electroactive species, if present, can be detected.
Of interest as general background are a number of publications identified as follows:
Oldham et al., Analytical Chemistry, Vol. 38, No. 7, June 1966, pages 867-872; Klein et al., "Improved Differential Pulse Polarography", Electroanalytical Chemistry and Interfacial Electrochemistry, Vol. 61, No. 1, May 10, 1975, pages 1-9. Christie et al., "Constant Potential Pulse Polarography", Analytical Chemistry, Vol. 48, No. 3, March 1976, pages 561-564. Parry et al., "Evaluation of Analytical Pulse Polarography", Analytical Chemistry, Vol. 37, No. 13, December 1965, pages 1634-1637; and Schmidt et al., Modern Polarographic Methods, Academic Press, New York, N.Y. (1963). A number of electronic systems have been developed and disclosed which effect processing of signals developed from electrochemical detection cells, including the Letters Patent to David et al., supra. Examples of such systems are shown and discussed in U.S. Pat. Nos. identified as follows:
______________________________________ U.S. Letters Pat. No. 3,420,764 Schlein January 7, 1969 U.S. Letters Pat. No. 4,253,847 Matson et al. March 3, 1981 U.S. Letters Pat. No. 4,321,322 Ahnell March 23, 1982. ______________________________________
The known methods, techniques and systems may be characterized as providing a limited specificity and sensitivity.